The demand for improved thread profile and connection designs is continually increasing with the demand for high-torque operations in the drilling of wellbores in the oil and gas industry. Thread damage due to galling or over-torquing is a common concern for threaded connections, especially in high-torque applications. In recent years, new downhole motor power sections have been developed that can produce torques greatly exceeding the torque capacity of known threaded connection designs.
In general, the torsional capacity and fatigue resistance of a threaded connection can be improved by maximizing the root radius of the thread profile in order to decrease stress concentrations in these critical areas. However, simply increasing a thread's root radius comes at the cost of decreasing the contact area of the thread's load flank, which increases the contact stresses during connection make-up, making the connection more susceptible to galling.
Another way to increase the torsional capacity of a threaded connection is to add a secondary make-up shoulder, forming what is known in the industry as a double-shouldered connection. However this subjects to the threads to increased axial loading, which also increases the likelihood of galling.
One example of a problematic threaded connection is the rotor connection of a downhole motor power section. In order to withstand the higher torque output of modern power sections, it may be necessary to employ a double-shouldered connection. Typical double-shouldered connection designs utilize straight threads in order to maximize the shoulder contact areas between the pin and the box.
However, when galling or other thread damage occurs in a straight rotor connection, it is commonly necessary for the rotor box to be replaced and stub-welded in order to repair the thread damage. The process of replacing and stub-welding the rotor box is roughly ten times more costly than the “chase and reface” repair process that can be performed to repair the same type of damage on a connection having tapered threads. A high-torque, tapered-thread alternative to the current straight connection would provide large savings based on the current frequency of rotor stub welding. Accordingly, there is a need for improved threaded connections that are capable of handling torques greater than the torque capacity of currently-used connections, and which can be chased and refaced in the event of thread damage.